How could the poet feel his mother's presence, even though she isn't there?
The poet feels his mother's presence through his senses, which activate memories of her during everyday moments. Even though she is not physically there, certain experiences deeply remind him of her.
He feels her presence auditorily when a tune of a song she used to hum while rocking his cradle hovers over his playthings. He also senses her olfactorily when the smell of shiuli flowers and the scent of the morning service in the temple come to him as the scent of his mother.
Finally, he feels her presence visually when he looks into the distant sky and feels the stillness of his mother's gaze spreading all over the sky.
Explanation
The question asks how the poet feels his mother's presence despite her absence. The context poem by Rabindranath Tagore reveals that the poet relies on his three senses: hearing (auditory), smell (olfactory), and sight (visual). In the first stanza, the auditory sense of a hovering tune reminds him of her. In the second, the olfactory sense of shiuli flowers and the temple service reminds him of her scent. In the third, the visual sense of looking at the distant sky makes him feel her still gaze. The answer synthesizes these three points directly from the text.